Abstract
The treatment of anodic porous alumina (APA) plates in heated water containing 8-hydroxyquinoline (HQ) produces crystalline tris(8-hydroxyquinoline)aluminum (Alq3) microbelts about 5–10 μm wide and 5–20 μm long. These microbelts were found to aggregate to form flower-like structures on the surface. X-ray diffraction studies indicated that the Alq3 microbelts are composed of an α-phase having a meridional structure. The Alq3 microbelts exhibited green photoluminescence with a peak at around 520 nm. Scanning electron microscope images and energy dispersive X-ray spectra showed that this reaction is induced at the APA surface by the reaction between the HQ and amorphous Al2O3 species.
1. Introduction
Since the pioneering report in 1987 by Tang and Van Slyke [1] on efficient green electroluminescence using tris(8-hydroxyquinoline)aluminum (Alq3), Alq3 has been adopted as an important electron transport layer as well as a vital light-emitting material for organic light-emitting diodes. The preparation and characterization of Alq3 have been widely studied [2–20]. Because of the bidentate ligand, Alq3 complexes form two isomers, as shown in Figure 1: meridional and facial. Interestingly, the two isomers exhibit different types of photoluminescence (PL): specifically, green [2, 3] in the case of the meridional isomer and blue [4–7] for the facial isomer. To date, five crystalline phases (α, β, γ, δ, and ε) have been identified for Alq3 [8–10]. To the best of our knowledge, α-, β-, and ε-Alq3 are formed by meridional isomers, while γ- and δ-Alq3 are formed by facial isomers [2, 5–7, 10–13].
(a)
(b)
Moreover, Alq3 crystals with a wide range of morphologies have been prepared, including nanoparticles produced by vapor condensation [14], nanowires produced using either a single-step heat treatment [15] or an adsorbent-assisted physical vapor deposition (PVD) process [16], pine needle-like crystals produced by PVD [3], and nanorods produced by solution synthesis with a surfactant [17]. All of these Alq3 crystals demonstrate interesting morphology-dependent properties. For example, the Alq3 nanoparticles prepared by vapor condensation were amorphous and the maximum PL intensity at about 538 nm increased as the average particle size decreased [14]. These same Alq3 nanoparticles can be heat treated and grown into the α-Alq3 nanowires via the γ-Alq3 phase [15]. When Alq3 nanowires were fabricated by heating mixtures of silica gel and Alq3, they exhibited a blue shift in the absorption spectra with decreasing nanowire diameter, and the photoluminescence vibrational fine structure intensified with increasing excitation energy [16]. Besides nanoparticles and nanowires, pine needle-like crystals with an ε-Alq3 phase were prepared by PVD in a double zone tube furnace [3]. Another α-Alq3 crystal morphology comprises nanorods, which have a regular hexagonal shape when prepared by self-assembly growth assisted by surfactants, and these exhibited suitable field emission characteristics [17]. A different fabrication technique in which 8-hydroxyquinoline (HQ) and Alq3 were dissolved in an ethanol/water solution and then exposed to high-intensity ultrasound irradiation resulted in the formation of well-defined Alq3 nanoflowers (α-Alq3) [18]. Since each particular structure of Alq3 exhibits unique optoelectronic properties that may be adopted for various nanodevice applications, it would be extremely interesting to explore additional novel fabrication methods capable of controlling the isomer formation, crystal structure, and/or morphology of Alq3 crystals.
In recent years, anodic porous alumina (APA) has attracted significant attention in various nanotechnology areas, including light-emitting devices [21]. As a result, there have been many studies of APA composites with highly fluorescent Alq3 [22–26]. In a previous study, we reported that Alq3 was formed on APA immersed in an ethanol solution of 8-hydroxyquinoline (HQ) at 60°C [27]. The PL spectra showed two peaks at wavelengths of 480 and 505 nm, which were attributed to isolated Alq3 inside the nanovoids and aggregated Alq3 in the nanopores of the APA, respectively. The formation mechanism of the Alq3 was considered to be due to the reaction of the HQ with the Al3+ ions in the oxide dissolution zone and the Al3+ ions adsorbed onto the surfaces of the pores and cracks.
In the present study, we continued to investigate similar reactions in boiling water containing HQ and APA and found that an α-crystalline-phase of Alq3 microbelts self-assembles on the APA plates in flower-like structures. This study characterizes the crystalline and optoelectronic properties of these unique structures.
2. Experimental
2.1. Preparation of APA Samples
An Al plate (99.59% purity, Nilaco Co., 1 cm2) was electropolished in a mixed solution of 60% perchloric acid and 99.5% ethanol (1 : 4 v/v) at a constant voltage of 20 V below 5°C for 40 s. Then, the APA sample was prepared by either one- or two-step anodization.
One-step anodization was carried out in 0.3 M H2SO4 at 20 V and at room temperature for 30 min. For the two-step process, anodization was carried out under similar conditions for 10 min. Then, the anodized plate was immersed in a mixed solution of 6.0 wt% H3PO4 and 1.8 wt% H2CrO4 at 70°C for 15 min. The plate was then anodized for a second time under the same conditions for 30 min.
2.2. Formation of Alq3 on APA Surface
The APA samples were placed in a beaker with distilled water containing HQ under various conditions (i.e., concentration, time, and temperature) and then dried in air at room temperature after being removed from the beaker. Observations of the appearance of the surfaces after drying were noted.
2.3. Instrumental Analysis
Scanning electron microscope (SEM) images and energy dispersive X-ray (EDX) spectra were observed by a Quanta 3D 200i scanning electron microscope (FEI) equipped with an EDX analyzer (EDAX). To observe a cross-section, Alq3 microbelts formed on a two-step APA surface were cut by a Ga+ focused ion beam. The X-ray diffraction patterns (XRD) were measured using a RINT 2200 (Rigaku) with Cu Kα radiation. The PL spectra were measured using an FP-8500 fluorescence spectrophotometer (JASCO) with an ISF-834 integrated sphere unit, excited at 370 nm.
3. Results and Discussion
3.1. Effect of Anodization Process on Crystal Morphology and Crystalline Phase
After APA was placed in a beaker with distilled water and HQ at various concentrations, times, and temperatures and then dried, visual observations of the surfaces of the APA were made. The upper APA surface was deep yellow (due to Alq3, which will be proved later), but the underside of the plate, where the APA was touching the beaker, was a paler yellow.
Figure 2 shows typical SEM images of crystals on the one-step (a) and two-step (b) APA surfaces treated in a 10 mM HQ solution at 100°C for 3 min. Belt-like structures, about 5 μm wide and 20–30 μm long, are formed by the one-step process. The microbelts appear to self-assemble into flower-like structures. For the two-step process, the belt-like structures were shorter, having a width of about 5 μm and a length of 10–20 μm.
(a)
(b)
Figure 3 shows the XRD patterns for the microbelts in Figure 2. The XRD patterns for both the one-step and two-step processes have essentially the same patterns and agree with those of the α-Alq3 powders previously reported by Brinkmann et al. [2], Braun et al. [4], and Kaji et al. [8], who confirmed that the microbelts consist of α-Alq3 regardless of the anodizing conditions. In a previous study, Katakura and Koide [10] reported that the treatment of boehmite, AlO(OH), with HQ in boiling water produces either α-Alq3 or γ-Alq3 depending on whether the reaction is allowed to continue for 1 h or 90 h, respectively. Tsuboi and Torii [28] also reported that the reaction of aluminum hydroxide, Al(OH)3, with HQ forms α-Alq3 and γ-Alq3 after a reaction time of 24 h and 100 h, respectively. Our XRD results agree with these previous studies at relatively short reaction times. Specifically, the reaction of APA with HQ at 100°C for 3 min leads to the production of α-phase Alq3, in a similar manner as the boehmite and aluminum hydroxide, after 1 h and 24 h, respectively. These findings suggest that the precursor of aluminum has little effect on the formation of the crystalline phases and/or isomers of Alq3 but that the reaction time is an important factor for inducing the isomerization reaction.
To spectroscopically confirm the phase of Alq3, PL spectra were measured for the microbelts formed on APA surfaces prepared under both anodizing conditions (Figure 4). PL peaks can be observed at around 520 nm, agreeing with the previous results for α-Alq3 made up of the meridional isomer [2, 4]. The results of the XRD and PL studies lead us to conclude that the microbelts consist of the α-phase of Alq3 without depending on the anodization processes.
3.2. SEM Observation of Alq3 on APA Surface
Figure 5 shows SEM images of the as-prepared one-step APA sample and the samples treated in the HQ solutions of different concentrations at 100°C for 10 min. A rough surface with splinters and cracks is observed on the as-prepared surface (a). With the 1 mM HQ solution (b), the surface becomes smoother than that of (a). Although no crystals can be observed on the surface, green fluorescence was observed when the surface was irradiated with ultraviolet light. These results suggest that, with this concentration, the APA surface reacts with the HQ. The Alq3 microbelts appear with the 3 mM HQ solution (c) and further increase with the HQ concentration, as shown in (d)–(f).
(a)
(b)
(c)
(d)
(e)
(f)
Figure 6 shows the PL spectra for the APA samples shown in Figure 5. The PL peak of the as-prepared APA sample (a) exhibits a peak at around 450 nm due to APA defects [29]. At 1 mM (b), the PL spectrum shows two peaks at 473 and 505 nm, which are the same results found in our previous study of APA treated in HQ in EtOH at 60°C [27]. The former and latter peaks are assigned to the isolated Alq3 species in nanovoids, while the aggregated Alq3 species are assigned to the nanopores in the APA. Given the similarity of the surfaces and the spectra, we believe that the entrapped states of the Alq3 on the APA of (b) are almost identical to those treated in HQ in EtOH at 60°C. Above 3 mM HQ (c)–(f), the PL peaks were observed at around 520 nm due to the microbelts assigned to the α-Alq3 phase, discussed above.
Figure 7 shows SEM images of the one-step APA samples treated in a 10 mM HQ solution at 100°C for different dipping times. When the sample was dipped into the solution and immediately removed, nanosized particles with a diameter of 300–400 nm formed (a), as were a very small number of microbelts with lengths of less than 5 μm and widths less than 3 μm (b). We observed many more particles than microbelts. Thus, it would appear that the nanoparticles aggregate into microbelts. At 10 s (c), the microbelts start to self-assemble into flower-like structures. Then, the microbelts grew rapidly with time, such that the APA surface was covered with microbelts at 60 s (d). Between 180 s (e) and 300 s (f), no increase in the size was observed. However, the surface of the microbelts formed at 300 s appears somewhat disordered.
(a)
(b)
(c)
(d)
(e)
(f)
Figure 8 shows SEM images of the one-step APA samples treated in 10 mM HQ solutions at different temperatures. At around 30°C (a), there are protruding crystals with a diameter of about 1-2 μm on the surface, which may be sprouts from the Alq3 crystals. Above 60°C (b)–(d), the flower-like structures can be clearly observed. This temperature dependence gives us some valuable insight. The Alq3 crystals formed in the aqueous HQ solution at 60°C, while no crystals were observed on the APA treated in a solution of HQ in EtOH. This difference is probably caused by the low solubility of Alq3 in water and the large reactivity of HQ with APA in water, relative to that in EtOH.
(a)
(b)
(c)
(d)
Another important point is that the Alq3 crystals form at 60°C. In general, the surface of APA is considered to be amorphous Al2O3 [30], which changes to a boehmite layer as a result of the treatment of the APA in water above 90°C [31]. The results of the present study indicate that the microbelts are produced by the reaction between the HQ and amorphous Al2O3 species of the APA without a boehmite layer.
3.3. Growth Mechanism of Alq3 Flower-Like Structure
We considered the growth steps of the Alq3 crystals formed from the HQ species, based on the sizes of the crystals observed in time-dependence studies (Figures 7(a)–7(c)): nanoparticles, belt-like structures, and flower-like structures. These steps appear to be similar to those of the sonochemical fabrication of Alq3 nanoflowers reported by Mao et al. [18] attributed to cavitation and an Ostwald ripening process.
We propose that the mechanism for evolution of the flower-like crystal morphologies observed in our study can be explained by Ostwald ripening (Figure 9); (1) the nanoparticles of Alq3 are formed on the APA surface as a result of the reaction of the HQ, (2) then the precipitated particles grow to form microbelts due to the aggregation of the nanoparticles, and (3) the individual microbelts assemble into flower-like structures.
Figure 10 shows cross-sectional SEM images (a) EDX mapping of the Alq3 microbelts on the two-step APA and (b) EDX spectra observed in regions I, II, and III. Region I only exhibits a peak for elemental Al, since it is detecting the original Al plate (a). Region II shows peaks for elemental O and Al (d), which is expected for the Al2O3 layer. On the other hand, C, N, O, and Al elements are only observed on the crystal surface, in region III, as shown in (e), suggesting that the Alq3 crystals exist mainly on the surface of the Al2O3. These results indicate that the formation of Alq3 occurs at the interface between the oxide film surface and the HQ solution.
(a)
(b)
(c)
(d)
(e)
4. Conclusion
Crystalline Alq3 microbelts, about 5–10 μm wide and 5–20 μm long, were formed on an APA surface treated in heated water containing HQ. The microbelts self-assembled into flower-like structures. The structure of the Alq3 microbelts is attributed to the α-phase, given the results obtained from XRD studies. These findings suggest that the precursor of aluminum has little effect on the formation of the crystalline phases and/or isomers of Alq3 but that the reaction time is an important factor for inducing the isomerization reaction. The green photoluminescence of the Alq3 microbelts peaks at around 520 nm. The crystalline phase and photoluminescence center of the Alq3 were formed on the APA independently of the anodization processes (i.e., one- or two-step anodization). The formation of the Alq3 species was induced by the reaction of the HQ species and amorphous Al2O3 species at the interface between the HQ solution and APA surface. The flower-like structures grew from the nanoparticles after first forming into microbelts, most likely due to Ostwald ripening. The results of the present study will provide a basis for devising further methods for preparing Alq3. To further investigate how Alq3 affects optoelectronic properties, we plan to prepare a facial isomer in the surface reaction of APA.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This research was funded by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2015-2016.